1. Field of the Invention
This invention relates generally to the manufacture of high quality glass and, more particularly, to optical quality glass used for optical lenses and glass sheet used for the production of TFT/LCD display devices that are widely used for computer displays.
2. Description of Related Art
Glass as melted from raw materials has many small bubbles of entrapped gases. These bubbles are considered defects in any glass product which requires optical properties. Bubbles of a size that can be seen by the eye or that interfere with the function of the product must be removed. The process for removing these bubbles is termed fining. Fining occurs after the glass is melted from raw materials, but before the glass is formed into a finished product. For optical quality glass this fining process is performed in a “finer” (or refiner), which is constructed of precious metal, typically platinum or a platinum alloy. The fining process is both chemical and physical. Chemicals are added to the glass such that the bubbles grow in size as they pass through the glass melting furnace and the finer. This invention is related to the physical aspect of fining, which is affected by the shape of the finer apparatus. The fining apparatus must be designed such that the removal of the bubbles from the molten glass is optimized. The finer is often very large, resulting in extremely high costs to fabricate because the glass contact surfaces are constructed of platinum or platinum alloy. In the fining process the bubbles rise to the top of the fining apparatus (finer) where they dissipate to the atmosphere. The size of the bubbles that are removed is a function of the size and design of the finer and the viscosity (fluidity) of the molten glass. In the glass industry these bubbles are called seeds if they are small (less than approximately 1 mm diameter) and blisters if they are large. Seeds are the primary concern as they are small in diameter and thus are more difficult to remove from the glass.
The glass seed entering the finer at the bottom of the inflow end of the finer must rise to the top of the finer at the outflow end where a vent to the atmosphere is located. The vertical speed of a seed in glass is inversely proportional to the glass viscosity, proportional to the square of the seed diameter, and proportional to the square of the glass density. The glass viscosity is a strong inverse function of temperature, therefore raising the glass temperature to a practical maximum increases the vertical speed of a given size seed. The detection of a seed in an optical product is a strong function of its viewable area, therefore we can use the diameter squared of a seed as the quality criteria. For a given glass the variation of the glass density in the fining process is a second order effect, thus we will consider primarily the glass viscosity and the seed cross-sectional area.
At the very high temperatures, approximately 1500° C., required to substantially reduce the glass viscosity, even the highest quality refractory materials are slowly dissolved by the glass. This introduces contamination and can also generate additional seeds in the glass.
In the prior art, a cylindrical platinum or platinum alloy (platinum herein) tube is used for all surfaces (walls) that contact the glass, such that the glass is not contaminated by the dissolution of refractory walls. The cylindrical tube is typically supported externally by refractory material (brick), which has the appropriate strength and insulating properties. The glass in the finer must be maintained at the required elevated temperature. Additionally, the glass entering the inflow end of the finer often must be heated to the desired fining temperature. This is done by either containment of the platinum and refractory finer assembly in heated (gas or electric) firebox or by electrical heating. The electrical heating of the finer is accomplished by either externally mounted electric winding (normally made of platinum) or the passing of electric current directly through the cylindrical platinum tube, thus using the electrical resistance of the tube to generate the heat.
The prior art design which has been typically used since the start of this practice in the first half of the twentieth century is a cylindrical platinum tube with and without internal baffles. The primary innovations to date have been in the design of the baffles to alter the flow path and to trap seeds for optimal seed removal. The prior art has included finer designs with and without an internal free surface.
FIG. 1A shows a simplified version of a cylindrical finer (1) as known in the prior art. FIG. 2A shows a cylindrical finer with baffles. In FIG. 1A, the molten glass (2) enters finer (1) at the inlet end (3) and flows out the outlet (4). There is a free surface, or vent (5), at the outlet end (4), which is connected to the atmosphere, to allow the seeds which accumulate at the top of the finer (1) to escape. FIG. 1B shows the typical path of seeds (7) in a finer with a cylindrical cross-section (1), which enter the glass inlet (3) entrapped in the molten glass (2). Shown are seeds which enter at the bottom of the finer inlet (3) and which must rise to the top of the finer at the outlet (4) so they may dissipate from the vent (5) to the atmosphere. With this cylindrically shaped finer (1) and with this size seed the length of the finer is such that the seeds just reach the top of the finer (1) where they are exposed to the vent (5) and can dissipate to the atmosphere.
The finer shown in FIGS. 1A and 1B has a diameter of 0.382 meters and a length of 2.5 meters. The glass flow rate is 7.41 metric tons per day. The viscosity is 100 poise. The seed diameter is 0.0007 meters. These parameters can be changed by normalizing using the equation:Q1*d12/η1=Q0*d02/η0 
where:
Q equals glass flow,
η equals glass viscosity, and
d equals seed diameter
In the baffled finer of FIG. 2A, the molten glass (2) enters the baffled finer (21) at the glass inlet end (23) and flows out the outlet (24). There is a vent (25) at the outlet end (24), which is connected to the atmosphere, to allow the seeds which accumulate at the top of the baffled finer (21) to escape. Some of the baffles (26) have holes (22) which are sized to distribute the flow of the molten glass (2) such that the average residence time for the glass as it flows through the baffled finer (21) is more uniform. Other baffles (28) are designed to move the flow path vertically. There is often a vent (29) in front of a baffle as baffles also trap the surface seeds into a foam-like accumulation which breaks down and dissipates into the atmosphere. FIG. 2B shows the movement of seeds (27) through the baffled finer (21). The baffles (26) and (28) make the paths of the seeds (27) in the baffled finer (21) quite tortuous. This allows the smaller seeds greater opportunity to coalesce together and form a larger seed, which in turn will rise faster.